1. Field of the Invention
The present invention relates to a vehicle headlamp provided with a discharge bulb having a ceramic light emitting tube. The light emitting tube has opposed electrodes and is filled with a light emitting substance.
2. Description of the Related Art
As shown in FIG. 13, a discharge bulb used as a light source of a vehicle headlamp includes an arc tube main body 1 formed by welding a shroud glass 4 to an arc tube 2 having a glass light emitting tube. The arc tube main body 1 is assembled to a synthetic resin insulating base 9 on a rear side thereof and is fixedly held thereby so that the arc tube 2 extends to a front side of the base 9. Specifically, a rear end side of the arc tube main body 1 is fixed to a front face side of the insulating base 9 by a metal piece 5, and a front end side of the arc tube main body 1 is supported by a lead support 6, which is also an electricity conducting path, extended from the insulating base 9.
The arc tube 2 includes a hermetically sealed glass sphere 2a filled with a light emitting substance (metal halide or the like) and a rare gas substantially at a center portion in a longitudinal direction of a glass tube. The end portions of the glass tube are sealed and include opposing electrodes. The arc tube 2 emits light by discharging electricity between the opposed electrodes. An outer side face of the shroud glass 4, which has a cylindrical shape and blocks UV light, is welded to the arc tube 2. The shroud glass 4 is provided with a light blocking film 7 for controlling a light distribution pattern of the arc tube 2. The discharge bulb forms a clear cutoff line by blocking a portion of light directed to an effective reflecting surface 8a of a reflector 8, thereby controlling light emitted from the arc tube 2.
However, the glass arc tube 2 (arc tube main body 1) poses a problem in that the filled metal halide causes glass tube to corrode. That is, the glass tube blackens and loses its transparency. Accordingly, the discharge bulb cannot achieve a proper light distribution pattern, and the service life of the glass tube is reduced.
Hence, as shown in FIG. 14, there has been proposed an arc tube 110 including a ceramic light emitting tube 120 (for example, see Japanese Patent Unexamined Publication JP-A-2001-76677, paragraph [0005] and FIG. 5). The arc tube 110 includes a ceramic, straight circular cylinder light emitting tube 120 that is sealed by cylindrical insulating members 130 at end portions 120a, 120a thereof, which form a hermetically sealed space filled with a light emitting substance and a rare gas. Electrodes 140, 140 are installed at opposing positions within the light emitting tube 120. The ceramic light emitting tube 120 is stable against the metal halide, and, therefore, the service life thereof is longer than that of a glass made arc tube.
However, the ceramic, straight cylinder type arc tube poses a problem in that its light distribution pattern has poor remote recognition because a hot zone of the pattern is considerably lower than the cutoff line.
That is, generally, a vehicle headlamp forms a dipped-beam (low-beam) light distribution pattern by using an effective reflecting surface that is provided at a position above the bulb. The effective reflecting surface is designed by projecting a light source images A, having rectangular shapes in correspondence with the light emitting tube 120, on a light distribution screen at a front side of the reflector with the rectangular shapes radially centering on a cutoff line/elbow portion. For example, a shape of the effective reflecting surface provided at a vicinity of a horizontal position in a left and right direction of the light emitting tube of the reflector is designed by projecting light along the cutoff line such that portions of light source images a contiguous in the lateral direction (direction along the cutoff line) and contiguous in a radial direction centering on the elbow portion overlap each other as shown by notations A, C in FIG. 15. The shape of the effective reflecting surface for forming left/right scattering light provided on an upper side of the effecting reflecting surface is designed by projecting light such that portions of the light source images a contiguous to each other in a lower direction or in a skewed direction constituting the radial direction centering on the elbow portion overlap each other as shown by notation B in FIG. 15. Further, the light distribution pattern shown in FIG. 15 is a light distribution pattern for a reflecting surface constituted by a paraboloid of revolution. Actually, light distribution patterns A1, B1, C1 having predetermined shapes without nonuniformities in light distributions as shown in FIG. 16 are formed by scattering the light source images a in a predetermined direction (mainly left and right direction) by forming a scattering step or the like at the reflecting surface.
However, a maximum brightness portion a1, which corresponds to the discharge arc, is disposed substantially at a center of the rectangular light image a, which has a width w. Therefore, there is a limit in designing the effective reflecting surface of the reflector so that a light distribution pattern includes a hot zone Hz proximate to a position of the cutoff line CL. That is, the position of the hot zone Hz is liable to be lowered relative to the cutoff line CL, which causes the light distribution pattern to have poor remote recognizability.
Further, a discharge bulb for a vehicle headlamp should have an excellent rise of a light flux so that a predetermined light flux is produced immediately after lighting. Therefore, a discharge bulb having a ceramic light emitting tube of a straight cylinder type, which is currently developed and disclosed in JP-A-2001-76677 or the like, uses of a light emitting tube with a tube diameter that is comparatively small (a volume of the hermetically sealed space is small) in order to improve a characteristic of rise of a light flux.
Therefore, the light source image a forming the light distribution pattern designated by notation B (B1) (that is, the light source image a projected radially in a lower direction or a skewed direction centering on the elbow portion) are rectangular shapes having a width that is not large because the diameter of the light emitting tube 120 is not large. Accordingly, the overlapping regions of the light source images a contiguous to each other near the elbow portion are small. Thus, a nonuniformity in color or a nonuniformity in a light intensity is conspicuous in the light distribution pattern, which causes poor front recognizability.